1. Technical Field
The present invention relates to a turbo compressor which is rotated by an induction motor via a speed-up gear mechanism, and more particularly to a turbo compressor which is rotated at a high but mechanically safe speed and which can be operated independently of a frequency of a commercial power source applied to the induction motor.
2. Background Art
Generally a high speed compressor includes a plurality of scroll-and-impeller sets (or a plurality of compressing units) connected in series. Each scroll-and-impeller set or compressing unit includes a scroll and an impeller rotatably placed in the scroll. A high speed compressor including two compressing units is generally called a two-stage compressor. The air is introduced into a first compressing unit and compressed therein. The compressed air is then introduced to a second compressing unit. In this manner, the air is successively compressed through the in-series connected compressing units and ultimately discharged from the last compressing unit.
A high speed compressor also has a gear box and a bull gear rotatably provided in the gear box. The compressing units are generally mounted on an outer surface of the gear box. A drive shaft extends from the bull gear out of the gear box and is driven by an induction motor. The drive shaft is directly connected with the induction motor.
Each impeller has a shaft extending into the gear box and a pinion gear is mounted on a free end of the shaft. The pinion gears of the impellers mesh with the bull gear inside the gear box. It should be noted that a single pinion gear may mesh with the bull gear inside the gear box if only two compressing units are mounted on the gear box and the two impellers share a common shaft and a common pinion gear.
The bull gear is a large gear as compared with the pinion gears in diameter and number of teeth. The bull gear is connected with the induction motor via the drive shaft. Accordingly, the impellers are driven by the induction motor via the full gear and pinion gears.
In order to raise a rotational speed of the impellers, the large-diameter bull gear and the small-diameter pinion gears are, directly engaged with each other like a planetary gear set. This is a one-stage speed-up mechanism.
Conventionally, a commercial power source is used for actuation of the induction motor. A frequency of the commercial power source is generally different from country to country or from region to region. Therefore, the rotational speed of the induction motor also differs depending upon a place where the induction motor is used. For example, when a two-pole induction motor is driven in a 60 Hz region, a rotational speed meter reads 3,600 rpm whereas when it is driven in a 50 Hz region, the rotational speed meter indicates 3,000 rpm.
A speed-up ratio by the bull gear and each pinion gear is generally about twenty times since a gear ratio between the pinion gear and the bull gear is about 1/20. Therefore, a maximum speed of the impeller reaches about 60,000-72,000 rpm.
Consequently, performances of the high speed compressor vary with a frequency of an available commercial power source.
In the meantime, a compact high speed compressor has been desired in the art. In order to reduce dimensions of the high speed compressor without degrading its performances, it is necessary to rotate each impeller at a higher speed such as some 100,000 rpm.
To raise the impeller rotational speed, the speed-up ratio and/or the motor rotational speed should be increased. Employing a larger bull gear and/or smaller pinion gears results in an increased speed-up ratio. In addition, using a two-stage speed-up mechanism can also raise the impeller rotational speed.
However, it is practically impossible to raise a gear ratio between the bull gear and the pinion gear while employing a one-stage speed-up mechanism. This is because if there is substantial difference between the two gears in dimension, the gear diameter of the bull gear becomes too large with respect to its gear tooth height and width so that mechanically appropriate meshing cannot be expected between the bull gear and the pinion gears as long as these gears are manufactured with ordinary tolerance. It is generally said that the practically feasible maximum speed-up ratio (or reciprocal of the gear ratio) is thirty times.
If a two-stage speed-up structure is utilized, the speed-up ratio at each stage can be reduced. However, an intermediate gear is needed, so that the overall speed-up mechanism becomes complicated and larger. This is contrary to the above-mentioned object, i.e., size reduction of a high speed compressor.
The rotational speed of the motor is determined by a frequency of a commercial power source since the induction motor is used. Thus, the rotational speed of the motor cannot be raised.
Another concern is that a discharge pressure of the high speed compressor becomes higher than a designed value in winter since an air temperature becomes lower in winter and therefore an air density is higher in winter than in summer. Consequently, the motor should bear a greater load in winter. To avoid this, conventionally an inlet valve of the high speed compressor is throttled.